An increasingly complex information-based society has generated the need for data storage media which offer high capacity, fast access and low cost. Optical memory media can potentially offer these advantages, however, the technology is not yet at an optimally developed stage. Researchers continue to discover new methods that reduce the cost and at the same time, improve the quality and storage capacity of optical media.
Optical memory media include discs, tapes and cards, among others. One embodiment of media is commonly referred to as a read-only memory (ROM). Data can be stored on ROM as extremely small structural relief features which are permanently molded onto the substrate surface. Subsequently, a reflective layer is adhered to the molded surface in order that the data can be read through differential reflection techniques.
Another embodiment of media is write-once-read-many (WORM), typically comprising one or more substrate layers and a recordable layer. Data is recorded on WORM by effecting an optically detectable change in the recordable layer using various techniques. These techniques include methods which cause a phase change, a topographical modification, a change in magneto-optical properties, a photochromism change, a bubble formation, or a selective ablation or melt (ablative/melt): Wen-yaung Lee, Journal of Vacuum Technology A3(3). pp 640-645 (1985).
The ablative/melt technique is the most commonly practiced method of recording data onto WORM media. Data is stored in the form of small pits or depressions in the surface of the recordable layer which are created by application of thermal energy, such as by using a focused beam of laser light. Exposure to laser light heats the exposed surface area to a threshold level at which point the layer melts and/or ablates away from the surface of the medium.
A layer suitable for recording using the ablative/melt technique commonly comprises an inorganic material such as a reflective metal. The thermo-sensitivity and conductivity properties of certain metals render them especially useful for laser light induced ablation. However, media which utilize a metallic recording layer, i.e., inorganic-based media, have disadvantages. The metallic layer is costly and difficulties are often encountered in depositing the metallic layer in a sufficiently uniform and secure manner on a substrate layer. Also, metallic layers are subject to environmentally induced chemical changes which may result in a change in optical properties.
In an effort to find an alternative to inorganic-based media, organics, especially polymers, have been used as a recordable layer. Organic-based media can be produced efficiently and are not as vulnerable to environmental attack as inorganic-based media. However, many polymeric recordable layers lack sufficient sensitivity to be of practical significance at the present time. In an effort to increase sensitivity to light energy, photosensitive dyes have been incorporated in organic-based media, i.e., dye-doped media. Organic dyes are chosen which exhibit high extinction coefficients near or at the output wavelength of incident laser light thus enhancing absorption of light energy. Unfortunately, high energy laser power, usually greater than 30 milliwatts (mWatts), is required to record data onto some of these dye-doped media.
Another recordable layer composition comprises a combination of organic and inorganic moieties. For example, Auerbach, U.S. Pat. No. 4,526,806, discloses a method of selectively depositing a pattern of reflective spots or lines of elemental metal or metalloid on a non-conductive substrate. As a layer containing a compound of the metal is exposed to high power laser light, the metal ion or metalloid compound dispersed in an oxidizable organic matrix is reduced to its metallic form.
It would be highly desirable to be able to produce an organic-based optical memory medium with improved sensitivity to laser light.